1. Field of the Invention
The present invention relates generally to cementing subterranean wells, and more particularly, to cement compositions which set into resilient solid masses having high strength.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of the well bore and the exterior surfaces of the pipe string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
The development of wells including one or more laterals to increase production has recently taken place. Such multi-lateral wells include vertical or deviated (including horizontal) principal well bores having one or more ancillary laterally extending well bores connected thereto. Drilling and completion equipment has been developed which allows multi-laterals to be drilled from a principal cased and cemented well bore. Each of the lateral well bores can include a liner cemented therein which is tied into the principal well bore. The lateral well bores can be vertical or deviated and can be drilled into predetermined producing formations or zones at any time in the productive life cycle of the well.
In both conventional single bore wells and multi-lateral wells having several bores, the cement composition utilized for cementing casing or liners in the well bores must develop high strength after setting and also have sufficient resiliency, i.e., elasticity and ductility, to resist the loss of the bonds between the pipe and formation and the cement composition. Also, the cement composition must be able to resist cracking and/or shattering as a result of pipe movements, impacts and shocks subsequently generated by drilling and other well operations. The bond loss, cracking or shattering of the set cement allows leakage of formation fluids through at least portions of the well bore or bores which can be highly detrimental.
The cement sheath in the annulus between a pipe string and the walls of a well bore often fail due to pipe movements which cause shear and compressional stresses to be exerted on the set cement. Such stress conditions are commonly the result of relatively high fluid pressures and/or temperatures inside the cemented pipe string during testing, perforating, fluid injection or fluid production. The high internal pipe pressure and/or temperature results in the expansion of the pipe string, both radially and longitudinally, which places stresses on the cement sheath causing it to crack or causing the cement bonds between the exterior surfaces of the pipe or the well bore walls, or both, to fail which allows leakage of formation fluids, etc.
Stress conditions also result from exceedingly high pressures which occur inside the cement sheath due to the thermal expansion of fluids trapped within the cement sheath. This condition often occurs as a result of high temperature differentials created during the injection or production of high temperature fluids through the well bore, e.g., wells subjected to steam recovery or the production of hot formation fluids from high temperature formations. Typically, the pressure of the trapped fluids exceeds the collapse pressure of the cement and pipe causing leaks and bond failure. Other compressional stress conditions occur as a result of outside forces exerted on the cement sheath due to formation shifting, overburden pressures, subsidence and/or tectonic creep.
Thus, there are needs for improved well cement compositions and methods whereby after setting, the cement compositions form highly resilient solid masses which have high compressive, tensile and bond strengths sufficient to withstand the above described stresses without failure.
The present invention provides improved cement compositions and methods for sealing pipe in well bores which meet the needs described above and overcome the deficiencies of the prior art. The improved compositions of the invention are basically comprised of a hydraulic cement, an aqueous rubber latex present in an amount in the range of from about 40% to about 55% by weight of hydraulic cement in the composition, an effective amount of an aqueous rubber latex stabilizing surfactant, and silica hydrophobicized with silicon oil present in an amount in the range of from about 0.5% to about 2% by weight of the composition.
The improved methods of this invention for cementing pipe in a well bore are comprised of the following steps. A cement composition of the invention is prepared which hardens into a highly resilient solid mass having high compressive, tensile and bond strengths. The cement composition is placed in the annulus between the pipe and the walls of the well bore and then allowed to harden therein.
It is, therefore, a general object of the present invention to provide improved cement compositions which harden into resilient solid masses having high strength and methods of using such cement compositions for sealing pipe in well bores.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.